I think this would be a great idea, but the suggestions forum is locked due to the 0.95 feature freeze. I think it would be great because I like playing as the FIB agents. I use the FIB Buffalo (which is a tahoe in my game) but I dislike the fact that backup is either cops or swat - do the FBI not have backup??? :P
Seriously, would the FBI ALWAYS call in local PD/Swat for every call? No. They would call in their own agents. I tihnk it would be a good thinng for LCPDFR too. Maybe ALT + F for FIVB backup, in the FBI vehicle carrying either combat pistol/shotsgun/uzi(maybe?).
I also think that Noose is WAY OP! Call in 1 unit and that pretty much cleans up any crime... FBI Backup should be between the cops and Swat. They don't carry super heavy weapons, but enough to get the job done.